Nondestructive testing is a procedure for determining the quality or characteristics of a structure without permanently altering the structure's properties. Examples include ultrasonic and radiographic inspection. In the avionics field, nondestructive evaluations of airplane components are performed to insure the structural integrity of the airplane.
In ultrasonic testing, ultrasonic transducers are used. Ultrasonic transducers convert electrical signals into mechanical vibrations in a test material and mechanical vibrations into electrical signals. Typically, ultrasonic transducers convert electrical signals into mechanical vibrations that propagate waves in the material to be tested via elastic deformations (the propagated waves are also known as elastic waves). The propagated waves interact with various features within the test material, such as flaws or defects. The ultrasonic transducers can also receive transmitted and reflected waves and convert the received waves into electrical signals. The received electrical signals can then be analyzed to determine if there are flaws or defects in the test material.
While different designs exist for ultrasonic transducers, a typical ultrasonic transducer utilizes a piezoelectric transducer. Piezoelectric transducers produce mechanical vibrations via the application of an electrical signal to an appropriate piezoelectric crystal or ceramic. In ultrasonic testing, the amount of electricity used to send a pulse is generally much greater than the signal received by the transducer. One drawback to ultrasonic testing is that when sending a pulse, unless the receiver is isolated from the transmitter, the voltage used to generate the pulse may saturate or overwhelm the receiver electronics, decreasing the recorded fidelity of the received echo signals.
Various solutions have been developed in structural health monitoring systems employing piezoelectric transducers to alleviate this problem. One approach is to employ multiple piezoelectric elements in a single ultrasonic transducer. For example, a transducer may include a piezoelectric ceramic element for transmitting the ultrasonic pulses coupled to transmitter electronics and a separate piezoelectric ceramic element for receiving echoes coupled to the appropriate receiver electronics. In other approaches, the transmitter does not act as a receiver, and any elastic energy is sensed only by other transducers within the structural health monitoring system.
One drawback is that providing separate transmit and receive elements increases the complexity, cost, weight and size of an ultrasonic transducer. In the area of medical ultrasound, single element transducers are used with a switch to isolate the receiver circuitry from the transducer drive signal. However, structural health monitoring (SHM) systems differ from medical ultrasound systems in several ways. First, the SHM systems generally operate at much lower frequencies than medical ultrasonic probes. In addition, a structural health monitoring system often uses a much larger number of sensors distributed and attached over a much larger area. The large number of sensors along with the finite power resources and weight limitations of an avionics environment favor the use of lower voltage drive signals along with simple transducers and transmit/receive circuitry.